1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device including a sapphire substrate (e.g., a sapphire wafer).
2. Description of the Related Art
The sapphire substrate is an insulating substrate. Unlike the silicon substrate (e.g., silicon wafer), in general, the sapphire substrate is not used alone in the semiconductor device fabrication. A semiconductor layer is provided on a surface of the sapphire substrate to form a composite semiconductor substrate (e.g., a composite semiconductor wafer), and the composite semiconductor substrate is used in the semiconductor device fabrication. For example, a silicon layer as a semiconductor layer is epitaxially grown on a surface of the sapphire substrate to form a silicon-on-sapphire (SOS) substrate (e.g., an SOS wafer), and then a semiconductor integrated circuit is fabricated in the silicon layer of the SOS substrate.
A lamp annealing technique is generally used in a rapid thermal annealing (RTA) process, in which short-time annealing is performed at 500° C. or higher, of the conventional method of manufacturing a semiconductor device using a silicon wafer. The lamp annealing technique uses a lamp annealing apparatus for heating an object by irradiating it with light emitted from the lamp. By directly irradiating the silicon wafer with light emitted form a tungsten halogen lamp, the silicon wafer absorbs the light, thereby being heated rapidly.
However, the SOS wafer is almost transparent to the wavelength of the light emitted from the tungsten halogen lamp and hardly absorbs the light. Therefore, the lamp annealing process by directly irradiating the wafer with light cannot increase the temperature of the SOS wafer and therefore cannot practically anneal the SOS wafer.
When annealing the SOS wafer, it is placed on a susceptor made of a carbon or ceramic material, and the susceptor carrying the SOS wafer is brought into a chamber of the lamp annealing apparatus. Heat is transferred from the susceptor heated by light emitted from the lamp to the SOS wafer. The indirect heating significantly reduces the temperature ramp rate, and therefore the gradual heating enables annealing of the SOS wafer.
The heat reaction treatment such as an activation process and a Salicide (Self-Aligned Silicide) process must be performed in a short period. In addition, the lamp annealing apparatus is an apparatus for a single-wafer and cannot put a long time into a temperature rise or temperature drop process. The temperature ramp rate should be at least about ½ to ⅕ of the temperature ramp rate of the RTA process for the silicon wafer without using the susceptor.
FIG. 1 is a diagram showing the process steps in the conventional method of manufacturing a semiconductor. device. In this case, the silicon wafer is heated for a thermal reaction treatment using the susceptor.
Referring to FIG. 1, in step S1 (PURGE), in order to purge a chamber of the lamp annealing apparatus, a nitrogen gas is introduced into the chamber for 30 seconds. In step S2 (RAMP), the temperature is raised from a room temperature to 500° C. at a temperature ramp rate of 20 degrees centigrade per second (° C./s). In step S3 (STEADY), the temperature of 500° C. is kept for 60 seconds to pre-anneal the semiconductor device. In step S4 (RAMP), the temperature is further raised to 850° C. at a temperature ramp rate of 20° C./s. In step S7 (STEADY), the temperature of 850° C. is kept for 30 seconds to perform the thermal reaction treatment of the semiconductor device. In step S6 (COOL), the temperature is lowered from 850° C. to the room temperature over 120 seconds.
In the conventional method using the susceptor as shown in FIG. 1, in consideration of the thermal capacity of the susceptor, the pre-anneal step S3 (STEADY) of keeping the temperature of 500° C. for 60 seconds is provided for the purposes of stabilizing the temperature of the susceptor and leveling out the temperature distribution of the susceptor, and the temperature ramp rate is lowered to 20° C./s, which is a low level for the lamp annealing apparatus and-is lower than the temperature ramp rate in the conventional method using no susceptor.
There is another method of manufacturing a semiconductor device using a sapphire wafer forms a thin film having a high thermal conductivity or a high light absorption on the back side of the sapphire wafer, so that heat is transferred to the sapphire wafer efficiently. Refer to the Japanese Patent Application Kokai (Laid-Open) Publication No. 10-70313, for example.
However, in the conventional lamp annealing of the silicon wafer using the susceptor as shown in FIG. 1, the actual time-varying temperature profile of the silicon wafer around 850° C. is sharp and has a quick follow-up capability to the specified temperature. In contrast to this, in the conventional lamp annealing of the SOS wafer, the actual time-varying temperature profile of the SOS wafer around 850° C. lags behind (i.e., causes an undershoot of) the temperature rise of the susceptor, thereby lowering the follow-up capability.
FIG. 2 is a diagram showing the time-varying temperature profiles of the center of the SOS wafer and of the susceptor in the conventional lamp annealing using the susceptor shown in FIG. 1. The temperature of the SOS wafer is detected by a special thermocouple provided on the SOS wafer. The temperature of the susceptor is detected by a pyrometer equipped in the lamp annealing apparatus.
In the lamp annealing apparatus, the pyrometer detects the temperature of the susceptor, the intensity of lamp output is controlled in accordance with the detected temperature, and the temperature is controlled accordingly. As shown in FIG. 2, the temperature of the susceptor can be controlled accurately, but the temperature of the SOS wafer reaches a specified temperature of 850° C. after a lag. The temperature of the SOS wafer continues to be about 40° C. lower than the temperature of the susceptor (specified temperature) for about 15 seconds. In the annealing step at the specified temperature of 850° C. for 30 seconds, the temperature of the SOS wafer is kept at the specified temperature for about 15 seconds actually.
Therefore, the shorter-period annealing cannot bring the SOS wafer to the specified temperature and cannot provide a desired effect of annealing even if the SOS wafer reaches the specified temperature after the lag in the temperature rise.
In this case, since the temperature of the susceptor is controlled as specified, it is difficult to improve the follow-up capability of the time-varying temperature profile during the temperature rise of the SOS wafer by changing the specified output intensity and/or illumination time of the lamp.
The definite causes of the lag in the temperature rise of the SOS wafer have not yet been found. However, it seems that the lag is ascribable to the sapphire wafer, and it seems that the causes of the lag are that the thermal conductivity of sapphire (42 W/m·K) is lower than ⅓ of the thermal conductivity of silicon (130 W/m·K), the thermal expansion coefficient of sapphire depends on the orientation of the crystal axis, the sapphire wafer has an internal stress.
It is also known that the sapphire wafer becomes warped when heated. The definite cause of the warp has not been found either. Possible causes include the following: Since the thermal conductivity of sapphire is up to ⅓ of that of silicon, there occurs a temperature difference between the front side and the back side of the sapphire wafer; the difference in the thermal expansion coefficient causes a local variation of the expansion coefficient when the temperature rises; and the internal stress cannot be suppressed because of the temperature rise. The warped sapphire wafer decreases the area contacting with the susceptor which is a heat source. The uneven contact between the susceptor and the sapphire wafer would interfere with heat transfer further and would further delay the temperature rise.
Further, when the sapphire wafer is placed on a hot plate heated in advance to heat the sapphire wafer at a high temperature ramp rate, the same phenomenon as described above would occur, thereby producing the same problems as described above.